Mass flowmeter

ABSTRACT

The invention relates to a flowmeter of the thermal type with a single flow sensor which is connected to control and temperature measuring means for measuring in a first measuring range by a first measuring method and in a second measuring range by a second measuring method. Detection means detect the measuring method to be selected on the basis of a flow measurement, and control means control the flow sensor in accordance with the selected measuring method.

[0001] The invention relates to a mass flowmeter of the thermal type.

[0002] Various mass flowmeters operating by the thermal principle areknown. In general, a flow of a fluid such as a gas or liquid flow, or atwo-phase flow, whose flowrate is to be measured is made to pass througha flow tube in the form a laminar or turbulent flow. When the flow tubeis locally heated, for example by means of a resistance wire coiledaround the tube, heat is transmitted through the tube wall to the gas orliquid flow by thermal conduction. The fact that the gas or liquid flowcarries along (transports) heat forms the basis for various methods ofmeasuring the mass flowrate.

[0003] A known mass flowmeter of the thermal type is described, forexample, in EP-A 1139073. This known mass flowmeter comprises a flowsensor in the form of a thermally conducting flow tube provided with aresistance wire coiled around the tube and acting as a heat source orheater and as a temperature sensor, in a first position, and with atemperature sensor located further upstream. A control circuit serves tokeep the temperature difference between the temperature sensors constantduring flowing, and the mass flowrate of the fluid flowing through thetube is determined from data of the control circuit. This method iscalled the constant temperature (CT) method. An alternative flowmeterutilizes the constant power (CP) measuring method, in which a constantpower is supplied to a heater centrally located on a flow tube, and thetemperature difference is measured between sensors arrangedsymmetrically with respect to the heater.

[0004] A disadvantage of such a measuring systems is that itsapplicability is restricted to a pre-defined measuring range. Adifferent flow tube and/or different heater windings are necessary foruse in a different measuring range.

[0005] The invention has for its object to provide a ‘universal’ massflowmeter that can be used over a wider measuring range than waspossible until now.

[0006] This object is achieved by means of a mass flowmeter of thethermal type which is characterized in that it comprises a single flowsensor which is connected to (power) control means and temperaturemeasuring means for measuring in at least two measuring ranges, in afirst measuring range by a first measuring method and in a secondmeasuring range by a second measuring method.

[0007] The term ‘flow sensor’ in the present context is understood todenote an assembly of a carrier with two or more heater elements andtemperature sensing means, which assembly is in a heat-exchangingrelation with a fluid flow to be measured during operation.

[0008] The invention is based on the recognition that a mass flowmeterof the thermal type can be constructed such that it is capable ofmeasuring by means of a single flow sensor in at least two mutuallyadjoining measuring ranges by two different measuring methods which canbe switched on as desired (possibly automatically).

[0009] A simple and practical embodiment of the flow sensor on the basisof the above is characterized in that it is provided with a heater H₁ ina first (upstream) position A and a heater H₂ in a second (downstream)position B, and with means for determining the temperature differencebetween A and B. The knowledge where one is in combination with a clevercontrol of the heaters renders it possible to adjust the measuring rangeas it were freely.

[0010] For certain applications, the carrier of the flow sensor isformed by a tube through which the fluid can flow and which carries twoelectrical windings internally or externally in positions A and B, thusforming the respective heaters H₁ and H₂. The electrical windings mayhave temperature-dependent resistance values and be included in acircuit for determining the temperatures at the locations A and B or thetemperature difference between A and B. An alternative possibility ofdetermining the temperature difference between A and B lies in the useof a thermopile which is in thermal contact with A at one side and withB at the other side.

[0011] For other applications, the carrier of the flow sensor is formedby a planar substrate, for example an IC or chip. It is possible toplace thereon, for example, two heater elements, for example in the formof conductor tracks that may or may not be meandering and between whichmeans are present for measuring a temperature difference between theheaters, such as a thermo-element or a thermopile.

[0012] Measuring methods suitable for use in combination with the flowsensor described above are in particular:

[0013] the TB (thermal balancing) method of measuring a flow from agenuine zero value over a given, low maximum range;

[0014] the CP (constant power) method of measuring a flow fromapproximately zero over a given, comparatively low maximum range;

[0015] the CT (constant temperature) method of measuring a flow from agiven threshold value (lower limit of minimum flow to be detected) up tovery high flow values.

[0016] The flowmeter according to the invention can be set such that themeasuring ranges associated with the above measuring methods aremutually adjoining or—preferably—have a slight overlap.

[0017] It is favorable to start in the TB mode upon switching-on of theflowmeter. If it is detected that the instantaneous flow lies outsidethe measuring range of the TB measuring method, a switch is made to theCP measuring method. If it is detected that the instantaneous flow liesoutside the measuring range of the CP measuring method, a switch is madeto the CT measuring method. The measuring ‘modes’ are thus sequentiallyswitched on. If a given signal value is undershot in a measuring range,a switch is made to the measuring method belonging to the next lowermeasuring range.

[0018] It is noted that the above does indeed describe a combination ofthe TB, CP, and CT measuring methods, but that other combinations may bepractical, depending on the envisaged application: for example, CP andCT; TB and CP; TB and CT.

[0019] A first embodiment is characterized in this connection in thatthe measuring and control means are designed for measuring by the TBmeasuring method so as to supply electric power to H₁ and H₂ inalternation and are provided with a control loop for controlling thetemperature difference between A and B down to zero in an iterativeprocess. All this takes place as described in U.S. Pat. No. 6,370,950.

[0020] A second embodiment is characterized in that the measuring andcontrol means are designed for measuring by the CP measuring method soas to operate H₁ and H₂ with constant power and to measure thetemperature difference between A and B.

[0021] A third embodiment is characterized in that the measuring andcontrol means are designed for measuring by the CT measuring method soas to measure the electric power to be supplied to H₂ necessary forkeeping the temperature difference between A and B at a constant value.

[0022] The basic principle of the invention: coupling of at least twomeasuring methods with different measuring ranges in one device may beimplemented in various manners.

[0023] These and other aspects of the invention will be explained inmore detail below with reference to a few embodiments and theaccompanying drawings. Corresponding components have been given the samereference symbols in the drawing, in which:

[0024]FIG. 1 shows a flow tube with two windings for a first embodimentof a flow sensor for a mass flowmeter according to the invention inperspective view;

[0025]FIG. 2A is a graph showing the relation between the output signalS=(P1−P2)/(P1+P2) and the flow φ in measurements by the TB principle;

[0026]FIG. 2B is a graph showing the relation between the measuredtemperature difference ΔT between two positions and the flow φ inmeasurements by the CP principle;

[0027]FIG. 2C is a graph showing the relation between the input power Pand the flow φ in measurements by the CT principle;

[0028]FIG. 3 shows a coupling of measuring ranges according to theinvention;

[0029]FIG. 4 shows a flow sensor arrangement with a planar substrate inplan view;

[0030]FIG. 5 is an operational diagram of an embodiment of the measuringsystem according to the invention;

[0031]FIG. 6 shows a flow tube with windings for a further embodiment ofa flow sensor according to the invention; and

[0032]FIG. 7 is an operational diagram of an alternative embodiment of ameasuring system according to the invention.

[0033]FIG. 1 shows a stainless steel flow tube 2 with an internaldiameter of approximately 0.8 mm and a wall thickness of approximately0.1 mm of a mass flowmeter for a fluid φ flowing through the tube 2 inthe direction of the arrow. The capacity of the tube 2 is approximately2 kg per hour for the calibration liquid isopropyl alcohol (IPA), and 1standard liter per minute for air. Resistance wires 3 and 4 of atemperature-sensitive resistance material, for example platinum ornickel or a nickel-iron alloy, of approximately the same resistancevalue by preference are coiled around the stainless steel tube 2 (in anelectrically insulated manner) such that they can each serve both as aheating resistance and as a temperature sensor. It is possible with thisflow sensor configuration to measure by the TB method, the CP method,and the CT method.

[0034] The stainless steel flow tube 2 conducts heat so well that it ispossible to measure the temperature of a medium flowing through the tube2 by means of the wire 3 wound around the tube 2 in an upstreamposition. An electric power P is supplied to the wire 4 wound around thetube 2 in a downstream position such that the temperature in thisposition will always lie above that of the medium in the first positionmentioned above by a given constant value ΔT (ΔT<5° C. for liquids,ΔT≦30° C. for gases). The electric power P to be supplied in order tokeep ΔT constant during the flow of the medium is measured and is ameasure for the flow. This measuring principle is known as the constanttemperature (CT) method. FIG. 2C shows the relation between the suppliedelectric power P and the mass flow φ when this method is used in aflowmeter of the kind depicted in FIG. 1.

[0035] The CT method has a wide range in upward direction, but below acertain minimum value φ_(min) of the flow this measuring method becomesinsensitive. At a value of P slightly above P_(min), for example atS=1.05 P_(min), therefore, a switch is made to the CP method, whichprovides a better sensitivity but a narrower measuring range. It ispossible, however, to have the latter merge into the range of the CTmethod. The operation is as follows: the two windings 3 and 4 each actboth as a heater and as a sensor, i.e. electric power is supplied toboth of them. This power is constant. The heat developed by the heatersdissipates through the tube wall. If there is no flow through the tube 2(φ=0), a symmetrical temperature distribution will arise across the tube2. If there is a flow through the tube 2, the upstream tube wall willcool down and the downstream tube wall will heat up. The temperaturedifference ΔT (=T₃−T₄) thus arising between 3 and 4 is detected by thetemperature-sensitive resistances 3 and 4 and is a measure for the flow.FIG. 2B shows the relation between ΔT and the flow φ. The maximummeasuring range is achieved when the upstream tube wall has cooled downfully and the downstream tube wall has heated up fully. A given tubewall thickness represents a certain sensitivity and a certain measuringrange. The sensitivity falls as the tube wall thickness increases (themaximum differential temperature becomes less because the heat ‘leakage’may be greater, but the measuring range increases: the minimumdetectable differential temperature does not occur until a higher flowis achieved). Measurable maximum values in the CP method are, forexample, gases up to 20 ml/min and liquids up to 2 g/hour.

[0036] A suitable point of transition to the measuring range of the CTmethod is, for example, at ΔT₆₀ (ΔT is 60% of ΔT_(top)). The point oftransition may be laid down in a microprocessor.

[0037] For very small flowrates, or as an alternative to the CPmeasuring method discussed above, the TB method described below may beused. This involves the use of a flow sensor in which the colder of thetwo heaters H₁ and H₂ is always supplied with power, and in which acontrol loop controls the measured temperature difference down to a zerovalue in an iterative process. The asymmetry of the power supply to theheaters for meeting the zero criterion is a measure for the flowrate.The TB method has an extremely stable and accurate zero point withoutoffset.

[0038] A suitable value to which the transition to the CP measuringmethod may be adjusted lies approximately at S=0.25 P_(tot) with the useof a tube as the carrier of the sensor components, where P_(tot) is thetotal power supplied to the heaters 3 and 4, i.e. P₃+P₄, andS=(P₃−P₄)/(P₃+P₄). For a planar substrate as the carrier it holds thatthe transition lies approximately at S=0.42 P_(tot).

[0039] The operation of the total measuring system according to theinvention will now be explained with reference to the operationaldiagram shown in FIG. 5. FIG. 5 is a block diagram of the operation of aflow sensor with a flow tube 2 through which a fluid to be measuredflows in the direction of an arrow φ. The tube 2 supports a winding ofresistance wire R₁ in an upstream location and a winding of resistancewire R₂ in a downstream location, each capable of functioning as aheater and as a temperature sensor. R₁ is connected to a measuring andcontrol block, which is a power control block, a source measurement unit1 (SMU 1) in the present case comprising a microcontroller μContr.1 anddigital-to-analog converter Dac1 for passing a controlled, known currentI₁ through R₁ (source portion), while the voltage drop V₁ across R₁ ismeasured by means of an analog-to-digital converter Adc1 (measurementportion). With these two data I₁ and V₁ and a calibration table(R_(T)=R₀(1+αΔT)) of R₁, the dissipated electric power P and (via thetable) the temperature are also known. R₂ is similarly connected to asource measurement unit 2 (SMU 2). The SMUs are connected to a(differential) control unit 6 which controls them and which generatesthe output signal S of the measuring system in dependence on themeasuring mode: TB, CP, or CT, i.e. S=(P1−P2)/(P1+P2); S=ΔT; S=P.

[0040] It is to be noted that FIG. 5 only shows three electricalcontacts per winding R₁ and R₂, but that preferably a four-contactmeasurement is carried out (so-termed Kelvin contacts).

[0041] An alternative to the measurement of the resistances of R₁ and R₂so as to determine their temperatures and thus their temperaturedifference is the use of a thermopile which is in thermal contact withthe region of R₁ at one end and with the region of R₂ at the other end.The use of such a thermopile TP is illustrated in FIG. 7. The voltagedelivered by the thermopile ΔP is read out in the source management unit3 (SMU 3), and the test result ΔT is fed to a control unit 8 comparableto the control unit 6 of FIG. 5.

[0042] A thermopile TP may be used to particular advantage where aplanar substrate 5 is used as the carrier for the heater elements H₁ andH₂ (FIG. 4). In that case the heater elements H₁ and H₂ may be providedon the substrate in the form of, possibly meandering, conductor trackson the substrate 5 with the thermopile TP in between. The substrate mayhave a very small thickness (for example <100 μm) and a small surfacearea (for example <5×5 mm) and is accordingly suitable for specialapplications. It may be mounted on a pin that is passed through the tubewall such that the measurement can take place locally inside the flowtube. The substrate 5 may be mounted on a foil with conductor tracks oron a PCB in order to make possible electrical connections to control andmeasurement circuits. This ‘chip’ version of the flow sensor renders itpossible to measure also outside the flow tube, indeed in any spacewhatsoever.

[0043] Since the implementation of the CP and TB measuring methods bothsupplies electric power to the windings (heater function) and measuresthe temperature (sensor function) in each of the (upstream anddownstream) measuring positions, it may be practical to separate the twofunctions, as is shown in FIG. 6.

[0044] In the flow sensor of FIG. 6, the upstream winding 3 of FIG. 1 issubdivided into two parts, each having a distinctive function, i.e. aheater part 3 in which power is dissipated and a temperature sensor part14 that detects the temperature of the heater and of the medium. The twoseparate windings 3 and 14 are preferably intertwined. It is importantthat both the temperature sensor 4 and the temperature sensor 14 can bemade from resistance wire originating from the same reel, so that theyhave identical temperature resistance coefficients. Their resistancevalues should preferably also be exactly the same. To give the heater 3as large as possible a heat-exchanging surface area, the resistance wirefor the heater 3 is preferably given a greater diameter than theresistance wire for the sensors 4 and 14.

[0045]FIG. 6 shows a temperature-sensing part 4 and a heater part 13also for the downstream winding 4. The same holds for this heater/sensorcombination as for the winding 3.

[0046] Instead of a set of wire windings coiled around the tube, forexample, a resistor pattern provided on a foil wrapped around the tubemay be used as the heater and/or sensor resistance. This may be providedinside the flow tube, if so desired.

[0047] A mass flowmeter according to the invention is applicable in manyfields, for example as a liquid flowmeter in combination with a controlvalve in a method of manufacturing glass fibers for telecommunication.The sensor measures and controls a flow of a silicon-containing liquidsuch as methyltrichlorosilane or TEOS. This liquid is brought into thevapor phase by means of a vaporizer. Silicon is bound to oxygen so as toform glass in a chemical vapor deposition (CVD) process. This glass, inrod shape, is subsequently drawn into long glass fibers while beingheated.

[0048] Another application is research and development in the field offuel cells. The sensor is used, for example, in combination with acontrol valve or a pump for supplying a fuel, such as methanol orgasoline, and water to the cell.

[0049]FIG. 3 diagrammatically depicts the relative locations of themeasuring ranges associated with the measuring methods to be switched onwithin the context of the present invention. If, for example, themeasuring methods CP and CT are to be switched on, and if it ispossible, for example, to measure from 1% to 100% in each of theirmeasuring ranges (range 1:100), then a switch-over from the onemeasuring method to the other one will give a total measuring range of1:10,000. Even wider ranges can be achieved within the scope of theinvention, for example in that the TB measuring method is additionallyswitched on for the lower flowrates. This is of particular importancefor medical, analytical, fuel cell, and natural gas applications.

[0050] The invention also covers modifications of the embodimentsdescribed above.

[0051] Thus the flow sensor may comprise further resistances in otherlocations on the tubular or planar carrier in addition to the twoelectrical resistances mentioned above.

[0052] The block diagrams of FIG. 5 and FIG. 7 show separatemicrocontrollers μContr.1, μContr.2, μContr.3. It is possible for onlyone microcontroller to be present in combination with a multiplexer thatoffers the signals to this microcontroller in a correct manner.

[0053] In the situation of a flow sensor whose electrical resistancesare provided on a planar substrate, these resistances may be made from atemperature-sensitive material and may be used for measuring thetemperature difference between them. A thermo-element or a thermopile isnot necessary in that case.

[0054] In the situation in which the electrical resistances are coiledas resistance wires around a tubular carrier, the means for measuringthe temperature difference may comprise a thermo-element or a thermopileprovided on a foil wrapped around the carrier. The foil may even lieinside the tubular carrier.

[0055] It is even possible to provide the resistors themselves (theheaters) on the foil and to provide the latter around (or in) thetubular carrier.

[0056] It is noted that flow sensor arrangements other than those shownin the embodiments may be used.

[0057] Thus, for example, a flow tube may be used with an upstreamtemperature sensor winding, a downstream heater/sensor winding, and aninterposed Peltier element. The Peltier element is used as a heaterduring measurements in the CP mode while the upstream and downstreamwindings serve as temperature sensors. During measurements in the CTmode the windings operate in the manner described above for the CTmethod, and the Peltier element is controlled such that it providescooling in the case of no flow or a weak flow so as to prevent anundesirable heating of the upstream sensor via the tube.

[0058] Summarizing, the invention relates to a flowmeter of the thermaltype with a single flow sensor which is connected to control andtemperature measuring means for measuring in a first measuring range bya first measuring method and in a second measuring range by a secondmeasuring method. Detection means detect the measuring method to beselected on the basis of a flow measurement, and control means controlthe flow sensor in accordance with the selected measuring method.

1. A mass flowmeter of the thermal type, characterized in that itcomprises a single flow sensor which is connected to control means andtemperature measuring means for measuring in at least two measuringranges, in a first measuring range by a first measuring method and in asecond measuring range by a second measuring method.
 2. A mass flowmeteras claimed in claim 1, provided with a heater H₁ in a first (upstream)position A and a heater H₂ in a second (downstream) position B, and withmeans for determining the temperature difference between A and B.
 3. Amass flowmeter as claimed in claim 2, characterized in that the heatersH₁ and H₂ are supported as electric windings of resistance wire by atube.
 4. A mass flowmeter as claimed in claim 2, characterized in thatthe heaters H₁ and H₂ are provided in the form of conductor tracks on aplanar substrate.
 5. A mass flowmeter as claimed in claim 2, wherein themeans for determining the temperature difference between A and Bcomprise a thermoelectric pile provided between the heaters H₁ and H₂and thermally coupled at one side to A and at the other side to B.
 6. Amass flowmeter as claimed in claim 1, characterized in that the controland temperature measuring means allow measurements by at least two ofthe following measuring methods: the TB (thermal balancing) method ofmeasuring flows, from zero; the CP (constant power) method; the CT(constant temperature) method of measuring flows from a given thresholdvalue up to high values.
 7. A mass flowmeter as claimed in claim 6,characterized in that the measuring ranges associated with the measuringmethods used at least adjoin one another.
 8. A mass flowmeter as claimedin claim 7, characterized in that the measuring ranges have slightoverlaps.
 9. A mass flowmeter as claimed in claim 6, characterized inthat the measuring ranges used are switched on in order of risingmeasuring range value during starting.
 10. A mass flowmeter as claimedin claim 6, characterized in that the measuring and control means aredesigned for measuring by the TB measuring method so as to supplyelectric power to H₁ and H₂ in alternation and are provided with acontrol loop for controlling the temperature difference between A and Bdown to zero in an iterative process.
 11. A mass flowmeter as claimed inclaim 6, characterized in that the measuring and control means aredesigned for measuring by the CP measuring method so as to operate H₁and H₂ with constant power and to measure the temperature differencebetween A and B.
 12. A mass flowmeter as claimed in claim 6,characterized in that the measuring and control means are designed formeasuring by the CT measuring method so as to measure the electric powerto be supplied to H₂ necessary for keeping the temperature differencebetween A and B at a constant value.
 13. A mass flowmeter as claimed inclaim 1, characterized in that it is provided with detection means forselecting the measuring method on the basis of a flow measurement.